Within the moulded and extruded rubber goods industry, certain applications benefit from the inclusion of reinforcement, typically in the form of fibres, wires, cables, textiles, or woven structures within the rubber matrix of the moulded or extruded rubber article. This permits good dimensional control of the moulded rubber goods so that anisotropic behaviour can be designed into the product to enhance performance in the field. One such example is in the production of rubber tires where the introduction of metal cord, usually steel cord, into the rubber matrix of the tire to provide structural strength and geometric stability.
In order for a reinforcement means to function well, it should have adequate performance in respect of temperature resistance (to allow processing), high elastic modulus (to resist excessive rubber stretch) and excellent adhesion to the matrix rubber (so that the final reinforced rubber article can behave as a true composite).
The following abbreviations are used throughout the description: EPDM—rubber prepared from ethylene-propylene-diene monomer; BIIR—brominated isobutene isoprene rubber, CR—chloroprene rubber, CSM—chlorosulphonated polyethylene rubber, HNBR—hydrogenated acrylonitrile-butadiene rubber, NBR—acrylonitrile-butadiene rubber, SBR—styrene butadiene rubber, NR—natural rubber and BR-butadiene rubber.
Brass-coated steel cords are presently used as the principal strengthening elements in regions of pneumatic tires such as the belt, carcass and bead. Since the development by Michelin in 1946 of the radial tire, brass-coated steel cord has become a common form of reinforcement and is used in around 98% of the world's passenger tires and 70%-80% of truck tires. Brass-coated steel cords allow the tyre manufacturer to optimize many physical parameters of the tyre, such as strength, stiffness, modulus, stability, and uniformity. Indeed, high stiffness of the belt region is essential for good tread wear, handling and low rolling resistance characteristics of the tyre.
The bond formed between brass-coated steel cord reinforcement structures and the rubber compound of a tire during vulcanization is an essential characteristic governing the performance and durability of car and truck tires. Brass-plating of the steel is a well-known method by which improved bonding can be achieved between rubber compounds and steel.
Bonding of rubbers as mentioned above, such as NR, to brass-plated steel cord is thought to take place as follows. During vulcanization, exposure of brass (copper and zinc alloy) to sulphur, the vulcanizing agent, creates a strong bond by the formation of non-stoichiometric copper sulphide. In the first stage, zinc sulphide forms slowly and the subsequent interstitial diffusion of copper ions through the layer is hindered because of the difference in ionic radii, with copper ions from the brass migrating more slowly than zinc ions. Zinc sulphide is capable of forming a distinct layer; nevertheless copper sulphide thickens as copper ions diffuse into this layer via lattice defects. The amount of copper sulphide present in the layer is directly related to the degree of sulphidation and it is essential to delay the cross-linking process long enough to build a copper sulphide layer of critical thickness for good adhesion. After many years of study in this field, it has still not been established exactly how the copper sulphide layer interacts with the rubber, but it is thought that because the copper sulphide layer is dendritic in form, high bond strength is achieved primarily by a tight, physical interlocking between this layer and the vulcanized rubber. Chemical cross-linking by Cu—S—NR bonds has also been proposed as part of the bonding mechanism.
Cobalt carboxylates may be used, alone or in combination with epoxy resin systems, to improve and maintain a durable bond at the rubber-brass interface. The use of such cobalt salts provides a bond between brass-coated steel and rubber that is considered as a bench-mark by which the tire industry assesses rubber-metal bond strength. U.S. Pat. No. 6,059,951 discloses a wire made of steel and coated with a zinc/cobalt alloy used for making composite elements of elastomeric material. Adhesion promoters such as cobalt neodecanoate, added to the elastomeric material, promote adhesion of the elastomeric material to the wire.
Prior art bonding systems for brass-coated steel cord, as detailed above, may limit the compositions and processes which may be used if good bond strength is to be achieved. For instance, in order to achieve good and durable adhesion between rubber and plated steel, the rubber compounds used must contain sulphur at a relatively high concentration in addition to a cobalt carboxylate as a bond-improver. Also the vulcanization rate within the rubber compound must be slow in order for the bonding layer to be formed in the so-called “scorch period” of the cure. This slow vulcanization step may give rise to a rate-limiting factor in the production process. Consequently, it is desirable to provide bonding methods which eliminate the need for such a slow curing step, so that production rates may be increased. It is also desirable to reduce the levels of undesirable and toxic materials, such as sulphur, zinc and cobalt in the rubber compound used in a tire, in order to avoid to improve safety during manufacture and to reduce toxic waste problems when tires are eventually discarded. The appropriate care has to be taken when handling cobalt carboxylates during mixing rubber compounds.
One of the methods for bonding a metal wire and rubber is to plate the surface of the metal with Cu/Zn to form a Cu—S-rubber bond in a resulting rubber composite. This method, however, has the following drawbacks because the surface of the metal wire is coated with Cu/Zn (see, for example, Stephen Fulton, et al., “Steel tire cord-rubber adhesion, including the contribution of cobalt” Rubber Chemistry and Technology (2004) Vol. 78, 426-457).
1) A CuxS layer overgrows under some vulcanization conditions and easily peels off the surface of the wire. As a result, the adhesion to rubber decreases.
2) The adhesion to rubber decreases under conditions of high temperature, high humidity, and high concentration of salt. (The difference in ionization tendency between Cu and Zn (Zn having a greater tendency to ionize than Cu) may form a ZnS layer at the interface between metal and rubber, resulting in a decrease in the adhesion to rubber.)
Hence there is a need for reinforcement structures for rubber articles, such as tires, which can be strongly bonded to a rubber matrix of the rubber article without suffering from some or all of the problems of prior art bonding methods.